FIG. 1 illustrates a vehicle powertrain. Heavy lines indicate mechanical power flow whereas thin lines indicate flow of transmission fluid. Dotted lines represent control signals. Engine 10 drives torque converter 12 which, in turn, drives gearbox 14. Gearbox 14 may adjust the speed and torque before transmitting the mechanical power to an output shaft. The gear ratio of gearbox 14 is selected by providing pressurized fluid to hydraulically actuated clutches. Pump 16, driven mechanically by engine 10, draws fluid from sump 18. Regulator valve 20 diverts some portion of the flow back to the sump in order to controls the pressure in line pressure circuit 22. Regulator valve 22 responds to a control signal from electronic controller 23 indicating the desired line pressure value. A component that acts in response to a control signal is called an actively controlled component. In response to commands from controller 23, valve body 24 routes the pressurized fluid to the torque converter circuit 26 and the appropriate clutch circuits 28 to establish the desired gear ratio in gearbox 14. Fluid exiting the torque converter goes into bypass valve 28. When the fluid temperature is below a threshold, bypass valve 28 routes the fluid directly to lubrication circuit 30. When the temperature of the fluid is above the threshold, bypass valve 28 routes the fluid through heat exchanger 32 before routing it to lubrication circuit 30. Components which do not require any control signals, such as bypass valve 28, are called passively controlled components. Fluid in the lubrication circuit provides lubrication to gearbox 14 and absorbs heat. The fluid then returns to sump 18.
When fluid is routed through first through one circuit and then through the other circuit, the two circuits are said to be in series. The flow rate through the first circuit is equal to the flow rate through the second circuit and the sum of the pressure drops across the circuits is equal to the pressure drop across the combination. When fluid is routed through only one of the circuits, on the other hand, the circuits are said to be in parallel. The pressure drop across the first circuit is equal to the pressure drop across the parallel circuit and the flow rate of the combined circuit is equal to the sum of the flow rates through each of the parallel circuits.
The transmission operates most efficiently when the fluid is at an optimal temperature. When the fluid is too cold, its viscosity is higher increasing parasitic drag. If the fluid gets too hot, the viscosity is too low resulting in increased leakage around the pump and elsewhere. This increased leakage reduces the pressure available from pump 16 reducing the torque capacity of the clutches within gearbox 14. If the fluid temperature remains high for a sufficient period of time, the friction characteristics of the clutches change and shift quality degrades. The temperature of the fluid is controlled by selectively routing the lubrication fluid through heat exchanger 32. When the fluid temperature is high, lubrication fluid is routed through heat exchanger 32 such that heat is dissipated either directly to the air or to an intermediate fluid such as engine coolant. When the fluid temperature is low, on the other hand, bypass valve 28 routes the fluid directly to gearbox 14 bypassing the heat exchanger and thus permitting the fluid to warm up. Note that, although regulator valve 20, valve body 24, and bypass valve 28 are illustrated in FIG. 1 as distinct components, some embodiments may integrate regulator valve 20 and bypass valve 28 into the valve body.
Most transmissions use positive displacement pumps. The volume of oil that is pressurized by the pump per unit time is dependent on the pump displacement and the engine speed. The torque required to drive the pump is dependent on the displacement and the pressure to which the fluid is pressurized. The power loss of the pump is proportional to the torque and the speed. Some transmissions utilize fixed displacement pumps. If the pump pressurizes more fluid than required at a particular time, the excess volume in discharged by regulator valve 20 with no reduction in either pump speed or pump torque. To reduce pump power loss and improve fuel economy, some transmissions utilize a variable displacement pump. The pump displacement is adjusted as fluid flow requirements change. When less fluid is required, the reduced pump displacement results in lower pump torque and reduced pump power loss.